Introduction Imagine you are looking out from a high building and that a large part of a town is spread out in front of you. The buildings are crowded together but you have no problem separating them from each other. You know which are close by and which are far away. People are walking on the pavement beneath and you know that they are people even though they are as small as ants. Streams of cars and buses drive by and even from your lofty position you have no trouble distinguishing between them – in fact, you can easily recognise different makes of car. A person walking by looks familiar. You recognise their walk and build. They look up and you confirm that it is a colleague.